listen

The listener can be added to an object once, and if it is added one more time, its key will be returned.Note Notice that if the existing listener is one-off (added using listenOnce),
it will cease to be such after calling the listen() method.

listenOnce

After the event is called, its handler will be deleted.
If the event handler being added already exists, listenOnce will do nothing. Note In particular, if the handler is already registered using listen(), listenOnce()
will not make it one-off. Similarly, if a one-off listener already exists, listenOnce will not change it
(it wil remain one-off).